[lld] [LLD][COFF] Introduce Symbol::getDefined helper. (NFC) (PR #151253)

[Martin Storsjö via llvm-commits]

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@@ -91,6 +91,14 @@ bool Symbol::isLive() const {
   return true;
 }
 
+Defined *Symbol::getDefined() {
+  if (auto d = dyn_cast<Defined>(this))
+    return d;
+  if (auto u = dyn_cast<Undefined>(this))
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mstorsjo wrote:

This (very) vaguely changes the logic from before - previously we tested `dyn_cast<Undefined>` before, while we now test them in different order. (In theory this could have a performance aspect, but that's not relevant.) What I wanted to say here, is that as long as only one of `dyn_cast<Defined>` and `dyn_cast<Undefined>` can succeed, i.e. they are mutually exclusive, then this should be fine - and this mildly rewritten form looks ok.

https://github.com/llvm/llvm-project/pull/151253